The present invention relates to electrolytes for the evolution of black finely dulled surface layers destined for working surfaces of modules consisting of light-weight material, especially titanium material, by applying the method of the anodic oxidation by spark discharge (ANOF-method). These electrolytes are long-term stable and the modules coated with them will guarantee an optimal application under specific operational conditions as well. From specialist and patent literature there is known a variety of electrolytes for coating light-weight material with surface layers by means of anodic oxidation by spark discharge, especially for coating metals with a unipolar conductivity, such as Ti, Ta, Zr, Nb or Al. In the DD-patent specification No. 221 762 and the DD patent specification No. 229 163 black or grey-black surface layers on light metals, such as Al, produced by means of the ANOF-method are described. It is a disadvantage of these solutions that electrolyte solutions are applied which do contain, in particular, fluorides in the form of NaF or NH.sub.4 F besides dihydrogenphosphate as NaH.sub.2 FO.sub.4, besides tetraborates in the form of dehydrated borax Na.sub.2 B.sub.4 O.sub.7 and besides chromates or different admixtures. The feasibility of inserting non-metallic (carbides, oxides) or metallic components by means of fluoride-containing, ANOF-method produced electrolytes as dispersing layers on titanium material as it is described in the DD-patent specification No. 156 003, involves, apart from the disadvantageous presence of fluorides in the electrolyte, the disadvantage of a slight solubility of the components within the electrolyte and all the other disadvantages typical for dispersing layers as an inhomogeneous structure or the like. The opportunity of blackening is not described therein. The DD-patent specification No. 257 275 refers to decorative coatings, e.g. on titanium material, which are produced by means of applying the ANOF-procedure and an electrolyte consisting of NaF, NaH.sub.2 FO.sub.4, Na.sub.2 B.sub.4 O.sub.7 and potassium hexacyanoferrate (II) K.sub.4 [Fe(ON).sub.6 ]. This solution involves the complex problem of environmental and health protection because the electrolyte employed has a toxic activity and is cyanide-containing. The black color is achieved merely by applying the hexacyanoferrate (II) which forms a titanium spinel which is simular to the black Fe-Al-spinel and merely satisfies decorative requirements.
Moreover, a number of black layers on materials, in particular on metals with a unipolar conductivity, such as Ti, Ta, Zr, Nb, Al, are known the radition-physical quantities of which--the optical absorption (.alpha.) and the thermal emission (.epsilon.)--have nearly equal values.
On the one hand, we have the opportunity of simply black-dying the surfaces of the solar collectors and, this way, to obtain a layer having the properties indicated above. On the other hand, it is as well feasible to employ diverse types of lakes to produce black layers with a .alpha./.epsilon.-ratio of about 1. It is a disadvantage of this solution that does not yield a finely-dulled layer. Additionally an especially problematic circumstance is the relatively high loss in weight of the lakes due to the release of volatile components. After being exposed to 125.degree. C. and 10.sup.-5 Torr over a time period of 24 hours the weight loss amounts to 3-5.4% for lakes which are currently usual. According to the standard ASTM E 595-77 the maximum weight loss due to the minimum release of volatile components amounts to 1% after being exposed to 125.degree. C. and 10.sup.-5 Torr for 24 hours. Currently, there is no lake coating which meets these requirements. Hence, no other lacquer system ensures a thermovacuum stability.
The DD-patent specification No. 236 978 describes absorption layers consisting of dark colored, chromium-doped oxide layers on metals with a unipolar such as Ti, Ta, Zr, Nb, Al, and which, as well, are produced by a fluoride-containing electrolyte including dihydrogenphosphate, tetraborate, and chromate, applying the ANOF-procedure. Such layers, on the one hand, will have a high absorptivity of radiation of .alpha.&gt;0.92 but on the other hand, they show, unfortunately, a surface structure of such a roughness to cause multiple reflections so that the incident radiation will transmit its energy in the form of heat to the absorption layer and the heat is, in turn, transferred to the collector body. In comparison with the optical absorption .alpha. there is achieved only a low thermal emission .epsilon..